Interpolation as the same word defines it, is the action of bringing one or more elements between the start and end of a succession of them.
When we talk about video, interpolation is key to the formation of sequences of frames to be played with some frequency, are perceived with the human eye as continuous movement concept.
A video camera captures images at an average speed of 30 frames per second.
Imagine conducting an experiment in which an individual projecting a video sequence recorded and played back at 30fps. Then we repeat the same scene but this time recorded and reproduced 1,200fps.
Do you think the individual can tell the difference between them?
Unfortunately, human vision is slow compared to that of some animal species such as cats and birds. The sequence of images reproduced at 30 frames per second is more than enough for our eye to retain part of the previous image when it is already recording the next and the merging perceives smooth transitions between them that create the feeling of movement, so that the fast 1.200 frames per second, only we could achieve together distinguish those frames with very similar structural characteristics, ie, the images more similar to each other.
The easiest way to understand the interpolation process is when we see how an animation program generates a sequence of frames based solely on benchmarks set by the user.
This method is known as animation keyframes and unlike the techniques used with physical clay models where each new movement has to be recorded frame by frame, computer graphics artists save a lot of time and work by establishing the main positions objects and relegating the work of generating intermediate images to the computer through interpolation.
In the example image you can see how in 3DS MAX, the initial and final positions of the dial are set, tracking a path is determined and the program automatically generates all intermediate positions.
As I mentioned earlier, the slow motion is generated by recording a scene faster and reproducing in a lower ratio of frames per second, however, sometimes we want to artificially generate this effect from a sequence recorded at normal speed.
In the comparative picture you can see how in the first few frames of the upper sequence, decreased speed in Adobe After Effects was applied to play at 600% of their normal relationship.
In the lower sequence, after setting the duration to 600%, was further applied an interpolation filter as you can see in every picture, it is responsible for artificially generated based on the similarities of the pixels in each frame of reference, projections intermediate steps to make the transition between each of the images it more smooth. This process of artificial intelligence achieves excellent quality slow motion even without a high-speed camera.